Photodetector having an integrated function for elimination of the effects of stray light

ABSTRACT

A photodetector for measuring the position of an incident light beam on an active surface area of the detector includes an inactive area and a concentric stray light absorbing area, both of which outwardly surround the light-absorbing active area. All light incident on the stray light-absorbing area and the inactive area will generate a photoelectric current which is conducted to an earth ground via an electrode in the stray-light area. An electrical signal from the active area representing a position of the measured light in the active area will be unaffected by any stray light incident on the detector externally of the active area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention concerns a device for measuring the position of alighting spot having an integrated function, in order to handle straylight.

2. Discussion of the Prior Art

A position-sensitive photodetector, PSD, consists of a semiconductorwafer having a doped active surface with electrodes positioned inmutually opposite sides. The doped active area forms a pn-junction inthe semiconductor wafer, and a resistive layer. An incident light beamwill generate a photoelectric current which is proportional to theintensity of the incident light. Also, the photoelectric current will bedivided in the resistive layer in the two electrodes linearly with thedistance from the respective electrode. By comparing the magnitudes ofthe currents the expression (I_(A) -I_(B))/(I_(A) +I_(B)) gives theposition of the light beam on the active surface of the detector. (I_(A)and I_(B) represent the currents from respectively electrodes A and B).This is true with a high degree of linearity when the lighting point isincident only on the active surface. On the other hand, if light isincident on the semiconductor wafer externally of the active surface(which normally is referred to as the inactive area) a photoelectriccurrent is generated also in this area and is registered in bothelectrodes. However, these currents are non-linear as well as slow. Inother words, the position signal becomes considerably more non-linearand also very much slower. Thus, the possibility of obtainingsatisfactory position measurements is impaired and sometimes even madeimpossible. In cases when the detector is used in applications wherestray light occurs, this a considerable problem.

When the stray light emanates from other light sources than themeasuring light it is possible, provided that the stray light intensityis within reasonable boundaries, to handle the stray light by means ofeither electrical or optical filters. An electrical filter is effectiveby screening off, in the subsequent signal processing, signals emanatingfrom light having a different modulation from that of the measuringlight. This method is adopted above all when a modulated light sourcehaving a comparatively high degree of modulation is used, such as 10kHz, and receiving low-modulated or non-modulated stray light from theenvironment, such as sun light or fluorescent light etc. Opticalbandpass filters operate by only letting through light havingwavelengths within a narrow wavelength range. This phenomenon is usedwhen the stray light has a different wavelength from that of themeasuring light but otherwise the same modulation (most oftennon-modulated).

When the stray light emanates from the measuring light itself, owing toreflections in the metal parts of the instrument, for instance, or has awavelength and a modulation close to that of the measuring light, theproblem becomes more difficult to solve. The only way to eliminate theeffects of stray light has hitherto been to screen off the stray lightfrom the detector in some way or other. This could be effected in twodifferent ways. Either a diaphragm is placed immediately above thedetector or by applying directly on the detector surface a meansscreening off the light from the inactive area; The diaghragm couldeither be in the shape of a thin metal foil which is placed immediatelyahead of the detector, or a diaghragm which is integrated into theprotective glass by means of thin-film technology. This method putsconsiderably demands on the mounting of the diaghragm, particularly inthe case of small-size detectors. In addition, diffraction phenomena mayoccur in diaghragm opening which thus generates stray light. The otherpossibility, to apply, in the processing of the very detector, somematerial screening off the light from the inactive areas, may beeffected through vapourization of a metal, gold or aluminium. Thedisadvantages are that the thickness required in order to obtainabsolute screening off of the stray light is not compatible with anormal semiconductor process. In addition, the presence of metal onthese surfaces generates problems concerning the electrical propertiesof the detector. The consequence frequently is high leak currents thatand growth thereof may come adrift. An absorbing paint, alternatively anepoxy, may be applied directly on the inactive detector area. This does,however, entail some obvious disadvantages in as much as on the one handit is difficult to achieve sufficient opacity in the paint/epoxy toprevent passage-through of stray light and on the other the workinvolved in applying the paint/epoxy is a pure manual task, possible tocarry out only when small quantities are involved, in addition to whichthe task is difficult to perform with the desired accuracy.

SUMMARY OF THE INVENTION

The invention aims at eliminating the effects of a stray light on theinactive area by doping, around the active surface 1, a further area 2which also forms a pn-junction. This area is connected to earth.

DETAILED DESCRIPTION OF THE DRAWINGS

The FIGURE is schematic view of the photodetector of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

All photoelectric current that is generated as a result of lightincident on the stray light receiving area 2 will be connected to earthand therefore will not affect the position signal from the activesurface 1. Also the photoelectric current formed by the stray lightincident on the inactive area 6 outside the stray light receiving area 2will be handled by the latter area 2 and be conducted to earth via itselectrode 4. The active area and the stray light receiving area must beseparated by an inactive area 5. When the latter is made as narrow aspossible, approximately 10 μm, which is a great deal narrower than ispossible with anyone of the methods mentioned above, the signal fromthis area 5 will have but a minimum effect on the position signal.

The invention is not in any way limited to the geometry illustrated inthe accompanying drawing figure but could be applied to optionalgeometries in 1-dimensional PSDs, 2-dimensional PSDs as well as inphotodetectors of various appearences and geometries, such as dual,4-quadrant, arrays and the like.

I claim:
 1. A position-sensitive photodetector for measuring theposition of an incident light beam having energy, comprising:asemiconductor wafer having a doped active surface and a pair of ends; aresistive layer on the active surface of the wafer; two opposedelectrodes on each end of said active surface; a stray-light areaarranged externally around the active surface, said stray-light area formeasuring stray-light contacting said photodetector and for preventingsaid stray light from affecting a position measuring signal of saidincident light beam, said stray light area comprising a doped areacontaining a pn-junction and a grounding electrode attached thereto, thesignal from said stray light area being grounded; a first inactive areaexternally surrounding said stray-light area and a second inactive areadisposed between said active area and said stray-light area, eachinactive area connected to said same ground as said stray-light area.